The invention relates to electrically conductive fluids and to their use. In particular, the invention relates to electrically conductive non-aqueous fluids and their use in oilfield operations, especially in development of or maintenance of a source of oil and/or gas.
In the process of rotary drilling a subterranean well, such as a hydrocarbon well, a drilling fluid (liquid) or xe2x80x9cmudxe2x80x9d is circulated down drill pipe or coiled tubing, through the bit, and up the annular space between the drill pipe or coiled tubing and the formation, to the surface. The drilling fluid performs a multiplicity of different functions, including cooling and lubrication of the drill pipe or coiled tubing and drill bit, removal of cuttings from the bottom of the wellbore to the surface, and, often, transmission of drilling or formation information from downhole to the surface by electrical telemetry.
The functions required can be achieved by a wide variety of drilling fluids which are formulated with and contain various combination of liquids, solids, and gases. In general, drilling fluids are classified according to the constitution of the continuous or external phase, i.e., as an aqueousxe2x80x94(water) base or based drilling fluid, or as a non-aqueousxe2x80x94(organic liquid) base or based drilling fluid, the latter often simply referred to in oilfield operations as an oil-based fluid.
Aqueous-base (water-base) fluids constitute the most commonly used drilling fluid type. The aqueous phase may be formed of fresh water, or, more commonly, of a brine. As a discontinuous or disperse phase, water-base fluids may contain gases or water-immiscible fluids, such as diesel oil, in the form of an oil-in-water emulsion, and solids including weighting materials, such as barite. Water-base fluids also typically contain clay minerals, polymers, and surfactants for achieving desired properties or functions.
However, in drilling water-sensitive zones, such as reactive shales, or where bottom hole temperature conditions are severe, or where corrosion is a significant problem, non-aqueous base drilling fluids are preferred. This preference exists notwithstanding that, as will be recognized by those skilled in the art, water or moisture is almost always present in a wellbore, and many xe2x80x9cnon-aqueousxe2x80x9d drilling fluids contain water or brine as a discontinuous phase in the form of a water-in-oil emulsion (or invert emulsion). Solid additives in non-aqueous-base drilling fluids are analogous to those of water-base fluids, with consideration given to solubility, and non-aqueous base fluids also contain additives for the control of density, rheology and fluid loss. If an invert emulsion is present, it may be formed by and stabilized with the aid of one or more specially selected emulsifiers.
Although non-aqueous-base drilling fluids are more expensive than water-base muds, their operational advantage and superior technical performance warrant their use in the particular situations mentioned. Unfortunately, many prior art non-aqueous-base drilling fluids are characterized by low electrical conductivity, i.e., high resistivity. This low electrical conductivity is a technical disadvantage for important wellbore fluid functions, e.g., in transmission of information from downhole to the surface by electrical telemetry, particularly in directional drilling, and in electrical well-logging and imaging operations performed during the drilling operation to determine, inter alia, the type of formation and the material in the formation. In the latter mentioned operations, some logging tools work on the basis of a resistivity contrast between the fluid in the wellbore and that in the formation. In the operation of such resistivity focused logging tools, alternating current flows from the tools through the formation between two electrodes. Accordingly, the fluids in the path of the electric current are the wellbore fluid, the wellbore fluid which has penetrated the formation medium (such as formation rock) under differential pressure, and the formation fluids. As will readily be appreciated by those skilled in the art, low electrical conductivity or high resistivity by the wellbore fluid affects logging and imaging results, and analysis of the logging and imaging results may require substantial compensation in calculating formation resistivity.
Accordingly, the use of resistivity logging tools has been limited primarily to instances where a water-based drilling fluid is used for the drilling operation because of the very low electrical conductivity of the non-aqueous base fluids. Even in the case where electrically conductive brine is dispersed in an oil phase, the discontinuous character of the brine droplets in the oil phase prevents or inhibits the flow of electricity. Indeed, the inability of such emulsions to conduct electricity (until a very high potential difference is applied) is used as a standard test of emulsion stability.
Attempts to make oil-based drilling fluids electrically conductive for the purpose of electrical logging have been made. U.S. Pat. No. 2,542,020; U.S. Pat. No. 2,552,775; U.S. Pat. No. 2,573,961; U.S. Pat. No. 2,696,468; and U.S. Pat. No. 2,739,120, all to Fischer, disclose soap-stabilized oil-based fluids comprising an alkaline-earth metal base dissolved in up to 10% by weight water. The patentee claims reduction of the electrical resistivity to below 500 ohm-m, which corresponds to an increase of conductivity to xcexa greater than 2000 xcexcS mxe2x88x921. Again, U.S. Pat. No. 4,012,329 discloses an oil-external micro-emulsion made with sodium petroleum sulfonate and a reported resistivity less than 1 ohm-m (xcexa greater than 1 S mxe2x88x921).
Difficulties in relation to the resistivity of non-aqueous-base fluids in oilfield operations or development or maintenance of a well are not confined to drilling operations. Logging, imaging, etc., may also be conducted during xe2x80x9ccompletionxe2x80x9d of the well, i.e., when the well is being prepared for development or production, using wellbore fluids referred to as xe2x80x9ccompletionxe2x80x9d fluids, or later, e.g., in a well xe2x80x9cworkoverxe2x80x9d, using wellbore fluids referred to as xe2x80x9cworkoverxe2x80x9d fluids. The completion fluid is provided or circulated in the wellbore while such operations as perforation or sand exclusion are conducted. In maintenance or renewal (redevelopment) of a well, a xe2x80x9cworkoverxe2x80x9d may be conducted to restore or improve production. This operation will similarly involve provision of or circulation of a specially formulated workover fluid in the wellbore, and may involve logging in the wellbore. As will be evident, the conductivity of these wellbore fluids is also important, and the high resistivity of non-aqueous base fluids restricts their utilization in these situations. Accordingly, non-aqueous base or based wellbore fluids (e.g., drilling, completion, and workover fluids) having increased or enhanced conductivity, as well as methods for their use, might have great commercial value. The invention is directed to the provision of such fluids and methods.
Accordingly, in one embodiment, the invention relates to a non-aqueous base wellbore fluid of increased conductivity containing an effective concentration of cations and anions from a water stable hydrophobic ionic liquid. More particularly, the invention relates to such non-aqueous base wellbore fluid comprising a non-aqueous base liquid containing an effective concentration of cations and anions from a water stable hydrophobic ionic liquid. The wellbore fluid of this embodiment of the invention thus comprises a fluid formed by blending a non-aqueous base liquid with an effective amount of a water stable hydrophobic ionic liquid. As understood herein, an xe2x80x9ceffective concentrationxe2x80x9d of the cations and anions is that concentration sufficient to raise the conductivity of the wellbore fluid to the desired level or degree. Similarly, an xe2x80x9ceffective amountxe2x80x9d of ionic liquid will be that amount of the ionic liquid necessary or required, when blending with the high resistivity organic liquid, to provide the concentrations of cations and anions required in the formulated fluid, i.e., in an xe2x80x9ceffective concentrationxe2x80x9d. Finally, the expression xe2x80x9cionic liquidxe2x80x9d, defined more fully hereinafter, is understood to include mixtures of such substances.
In a second, preferred embodiment, the invention relates to a non-aqueous base wellbore fluid of increased conductivity comprising a single phase mixture of a non-aqueous base liquid and a minor amount of a second, different organic solvent liquid, or mixture thereof, soluble in the non-aqueous base liquid, containing an effective concentration of cations and anions from a water stable hydrophobic ionic liquid. The non-aqueous base wellbore fluid of this second embodiment of the invention is formed by blending a non-aqueous base liquid with a minor amount of a second, different organic solvent liquid, or mixture thereof, which is soluble in the non-aqueous base liquid, containing cations and anions in effective amount from a water stable hydrophobic ionic liquid.
The invention further comprises the utilization of the non-aqueous base wellbore fluids described in particular wellbore operations. In particular, the invention includes methods of drilling a well, of completing a well, and of well workover, as described previously, each of which is characterized by utilization of the non-aqueous base fluids as described herein.
The common denominator of each embodiment of the invention is that each involves the employment or use of one or more of a class of compounds generally referred to as xe2x80x9cionic liquidsxe2x80x9d. As understood herein, an xe2x80x9cionic liquidxe2x80x9d is a compound which is a liquid at ambient temperatures and which consists entirely of a cation and an anion or a substance which is a liquid at ambient temperatures and which consists entirely of cations and anions. Accordingly, an xe2x80x9cionic liquidxe2x80x9d is to be distinguished from a (1) xe2x80x9cmolecular liquidxe2x80x9d (e.g., carbon tetrachloride), and (2) a xe2x80x9csolutionxe2x80x9d which contain dissolved electrolyte (e.g., Na+Clxe2x88x92 in water). In the case of a quantity of an ionic liquid, the entire liquid is composed of cations and anions, i.e., it is a homogeneous liquid having those two components (hence also the expression xe2x80x9cmolten saltsxe2x80x9d). Thus, pure water would not qualify as an ionic liquid since it consists almost wholly of H2O molecules, rather than ions. Nor would Na+Clxe2x88x92 solution qualify as an ionic liquid, since it is composed of H2O molecules and Na+ and Clxe2x88x92 ions.